1. Field of the Invention
The present invention relates to chrominance signal separating apparatus for separating a chrominance signal from a composite color television signal.
2. Description of the Prior Art
Recently, progress of digital signal processing technique accompanies that an adaptive type chrominance signal separating apparatus which selects a plurality of filters according to existence of correlation between an arbitrary reference signal and the signals at the vicinity of the position of the reference signal in a horizontal or vertical direction on the screen in a chrominance signal separating apparatus which separates a chrominance signal from a composite color television signal.
FIG.1 is a block diagram of a chrominance signal separating apparatus in accordance with the prior art.
A band pass filter 301 extracts a chrominance signal component from a composite color television signal (hereafter, called VBS signal, which means video burst signal). Each one of 1H delay devices 302 and 303 delays a signal by 1H period, where 1H means one horizontal period. In digital circuits, memory devices are usually used as delay devices. A comb filter 304 separates a chrominance signal from the output signal of the band pass filter 301. A comb filter 305 separates a chrominance signal from the signal delayed from the output signal of the band pass filter 301 by 1H period and the signal delayed from the output signal of the band pass filter 301 by 2H periods.
An adder 306 adds the output signal of the band pass filter 301 and the output signal of the delay device 302 which is delayed from the output signal of the band pass filter 301 by 1H period. An adder 307 adds the output signal of the 1H delay device 302 which is delayed from the output signal of the band pass filter 301 by 1H period and the output signal of the 1H delay device 303 which is delayed from the output signal of the band pass filter 301 by 2H periods.
Absolute value circuits 308 and 309 output absolute values of the output signals of the adders 307 and 306, respectively. Low pass filters 310 and 311 filter the output signals of the absolute value circuits 308 and 309, respectively and output their average values. Comparators 312 and 313 compares the output signals of the low pass filters 310 and 311 with an arbitrary threshold value and judges which correlation is stronger. A smaller average value means stronger correlation. A mixing circuit 314 mixes the output signals of the comb filters 304 and 305 according to the output of the comparators 312 and 313.
A chrominance signal is extracted from a VBS signal at the band pass filter 301. The extracted chrominance signal component is applied to the delay devices 302 and 303 in order and one input signal to the delay device 302 and the two output signals from the deley devices 302 and 303 are applied to the two comb filters 304 and 305. Because each comb filter 304 and 305 is supplied with two signals which have an 1H period time difference, the output of each comb filter becomes a chrominance signal. While, the two signals which have an 1H period time difference are added at the adders 307 and 306, the sums are outputted to the following absolute value circuits 308 and 309, the absolute value signals are filtered at the low pass filters 310 and 311 and average values of the absolute values are outputted from the low pass filters 310 and 311, respectively. These values are vertical correlation values. The vertical correlation values are compared with an arbitrary threshold value k' at the comparators 312 and 313. Here, if the correlation value is smaller than the threshold value k', it is judged to have correlation. The mixing circuit 314 mixes the outputs of the comb filters 304 and 305, based on the outputs of the comparators 312 and 313. For example, an optimum chrominance signal is obtained by adopting the output of the comb filter which has correlation if there is some correlation in either output, or adopting the average value of the outputs of the two comb filters 304 and 305 if there is no correlation in both outputs.
However, the above-described configuration has a problem which misjudgment occurs in correlation judgment when the chrominance signal level is small, because the threshold value to be compared with the correlation value is fixed to k'.